Because of excellent compatibility with human bone, artificial bone made of apatite can be directly connected to human bone. Accordingly, artificial bone made of apatite has recently been appreciated for effectiveness, finding clinical applications in cosmetic surgery, neurosurgery, plastic surgery, oral surgery, etc. However, apatite is not necessarily completely identical with human bone in terms of mechanical properties and physiological properties. For instance, an artificial bone made of calcium triphosphate, apatite, etc. is harder and more brittle than human bone. Accordingly, it is difficult to form an artificial bone adapted to an implant site, and the artificial bone is easily detached from the implant site. In addition, while human bone repeatedly undergoes metabolism of absorption and regeneration, artificial bone made of apatite is not substantially dissolved or absorbed in a human body, so that the artificial bone remains in a human body semi-permanently, and that human bone does not grow uniformly in the implanted artificial bone. Accordingly, the remaining artificial bone breaks human bone at an interface with the human bone, making it likely to cause bone fracture.
Research has recently become active on artificial bone decomposable in the human body, which is closer in composition to human bone than the artificial apatite bone, and various proposals have been made. For instance, porous bodies having networks comprising hydroxyapatite, to which collagen and other binders, if necessary, are bonded, were proposed (Japanese Patent 3048289 and JP 11-513590A which is a family member of U.S. Pat. No. 5,776,193). Because these porous bodies are decomposable in a human body, human bone is formed in the porous bodies, and the porous bodies per se are absorbed in a human body. Accordingly, these porous bodies can be used for fixation of vertebra, filling of bone defects, repair of fractured bone, and grafting to jaw bone defects, etc.
The use of a porous body containing a calcium phosphate ceramic, particularly porous apatite having properties close to those of human bone, as a material to be implanted in a living body, provides little likelihood of being rejected by the host, and enjoys a short recuperation period because of high bone-forming capability. Cells participating in the formation of bone are induced in an artificial bone implanted in a bone defect portion, so that human bone grows while the implanted artificial bone is absorbed by the host, resulting in the replacement of the artificial bone by the grown bone tissue.
It has been found, however, that when the implanted artificial bone comes into contact with a soft tissue such as a gum, a subcutaneous tissue, muscle, an internal organ, etc., cells, fibroblasts, etc. in the soft tissue enter into the porous artificial bone, hindering the growth of a bone tissue.